Corrosion protection method



2,756,162 CORROSION rnornc'rloN METHOD No Drawing. Application March Serial No. 341,614 Claims priority, application Great Britain June 6, 19 52 11 Claims. (Cl. 117-62) I 1 lass This invention relates to an improved process for the protection of metallic surfaces' More particularly, it refers to a process for protecting aluminum coated corrodible metals from corrosion.

Metals which, in ordinary environments, are susceptible to corrosion are commonly protected by forming on the metal surface to. be exposed a coating of aluminum. 'It is believed that the protection thus afforded is at least in part due to the avidity with which aluminum combines with oxygen, particularly at elevated temperatures, and forms alumina on the aluminum coating. The latter is highly resistant to attack by corrosive influences, particularly by oxidizing atmospheres, and therefore materially prevents deterioration of the underlying metal. Aluminuin coated metals, particularly ferruginous metals, such as iron and steel, coated with aluminum, therefore, find extensive use for furnace parts, such as conveyors, stokers, toasters, kiln parts, heat treatment boxes, pyrometric equipment, in diesel engine construction, in equipment such as tube stills, valves, retorts and condenser parts in the oil refining industry and in storage as well as other vessels used for some corrosive chemicals.

'As mentioned above, aluminum coated iron and steel find extensive. application in the construction of equipment which is to be subjected tohigh temperatures. The present trend in furnace and combustion chamber design towards higher operating temperatures has led to considerable metallurgical problems particularly where a residual oil fuel is used. At temperatures above about 650 C, the ash formed on combustion of such fuel may corrode certain types of metallic components on which it is deposited.

Several methods for producing aluminum coated metals are available. In one method, akin to galvanizing, the coating is applied by dipping thecorr'odible metal into molten aluminum. Calorising, a process in which aluminum is diffused into the surface of iron, copper or other easily oxidizable metals, is also used. It isusually eife'cted by heating the metal in an atmosphere of hydrogen in a slowly rotating drum containing a mixture of aluminum powder, alumina and a small proportion of ammonium chloride, to a temperature above the melting point of aluminum. The calorising process may be carried out on metal which has been coated with aluminum byhot dipping. Upon heating, the aluminum diffuses into the .metal base producing a coating which is essentially the same as that obtained by calorising alone.

' A metal spraying process which, although giving a less durable coated product than that produced by calorising, is nevertheless extensively used because of the low initial cost and correspondingly low cost of renewing worn parts, consists in atomising molten aluminum by compressed air or by some mechanical 'device, forcing it through a nozzle at high velocity and allowing the resulting spray to impinge on the surface to be coated. The individual droplets of molten aluminum become covered byaf superficial 'film of aluminum oxide and are ilattened on impacting the surface to be protected so that the sur- States Patent 0 2 face is, in effect, covered by a series of tiny overlapping scales.

The effectiveness of aluminum coatings in preventing corrosion of the metal base is dependent to a considerable extent on the permeable porosity of the coating, by Which term is meant the extent to which irregularities in the coating provide a direct link between the basis metal and the external corrosive influences. Aluminum coatings produced by commercial processes in general exhibit permeable porosity'to a greater or lesser degree. lttends, however, to be particularly marked in sprayed coatings which depend for their effectiveness on the thickness and regularity with which the spraying process has been carried out. Even in high quality work, minute interstices may 'occur between the separate aluminum flakes through which a corrosive medium can penetrate to the metal base. a

It is an object of the present invention to improve the corrosion protection of metal articles. It is another object. of this-invention to substantially correct the corrosion permitted by the porosity of aluminum coatings. It is a particular object of this invention to substantially seal the pores present in aluminum surfaces. Other objects will become apparent during the following discussion.

Now, in accordance with the present invention, corrodible metal articles may be protected from corrosion by applying thereto a surface of aluminum or aluminum oxide, which is more or less porous, and thereafterapplying to said surface an aluminum alcoholate, said alcoholate being converted 'to an aluminum oxide, which may be1either alumina or aluminum hydroxide. More particularly, the present process comprises providing an aluminum surfaced metal article with a substantially continuous coating of aluminum alcoholate which is thereafter converted by hydrolysis or pyrolysis to alumina or aluminum hydroxide in a substantially continuous film thereby sealing the pores normally present in the aluminum surface. The term, aluminum alcoholate is understood to mean a compound of aluminum with alcohols as defined hereinafter as well as with phenols.

The aluminum alcoholate may, if sufliciently volatile, be applied to the surface to be treated in the form of vapor by the vacuum evaporation technique. Alcoholates which can be applied in this way are the alcoholates derived from aliphatic alcohols having less than five carbon atoms per molecule. The undiluted liquid aluminum alcoholates may also be brushed or sprayed on the surface to be treated. Preferably, however, the aluminum alcoholate is applied in solution in a volatile solvent of low surface tension which facilitates its penetration into the interstices of the coating and then evaporates leaving the aluminum compound filling the pores. Suitable solvents for use in the compositions of the invention are the aromatic hydrocarbons (preferably having less than eight carbon atom per molecule), such as benzene, toluene or the alcohols such as methanol, ethanol or isopropanol, the lower aliphatic xylenes, the lower aliphatic monohydric ketones such as acetone, methyl ethyl ketone or methyl isobutyl ketone and the lower aliphatic ethers, such as diethyl ether and di-isopropyl ether. The solutions may be brushed on, or applied by spraying. The aluminum alcoholate deposited in the pores of the coating decomposes either by hydrolysis, for example, in moist air, to aluminum hydroxide, or by the influence of heat on either the aluminum compound or on its hydrolysis product to alumina. At ordinary temperatures, the aluminum oxide thus produced acts as a mechanical barrier to the passage of water or aqueous solutions and thereby enhances the protection afforded by the aluminum coating. When used at high temperatures, e. g. in furnaces, the aluminum oxide, in a similar manner, prevents the ingress of corrosive media mechanically, and may also react with cyclic.

Examples of suitable monohydric alcohols are methanol, ethanol, isopropanol, butanol, tertiary-butanol,

hexanol, isohexanol, heptanol, tertiary octanol, Z-ethylhexanol, 2,5-dimethyl heptanol, dodecanol, tetradecanol, pentadecanol, octadecanol, cyclohexanol, methylcyclohexanol, furfuryl alcohol or benzyl alcohol. Phenol, o-cresol, m-cresol, p-cresol, commercial cresol mixtures, the xylenols and alphaand beta-naphthols are examples of suitable monohydric phenols.

Examples of suitable dihydric and polyhydric alcohols are ethylene glycol, diethylene glycol, propylene glycol, glycerol, sorbitol and mannitol. Suitable dihydric and polyhydric phenols are resorcinol and pyrogallol.

The aluminum alcoholates of the invention may also be prepared from mixtures of alcohols suchas are obtained, for example, by the hydration of mixtures of olefins obtained in cracking petroleum fractions, or by reaction of such olefins with carbon monoxide and hydrogen in the x0 process.

It is desirable that the aluminum alcoholate used in the process of the invention should contain a high ratio of aluminum to organic radical in the molecule in order that a sufficient quantity of aluminum hydroxide or alumina will be formed therefrom after application to the aluminum and/ or aluminum oxide coated metal without the necessity of employing large amounts of the aluminum compound. It is therefore desirable that the alcohol or phenol from which the aluminum compound is derived should be of relatively low molecular weight and the lower aliphatic alcohols are therefore preferred for this purpose. Low molecular weight alcohols are also desirable where it is not intended that the surface treated with the aluminum alcoholate be subsequently heated, since it will then be necessary for the alcohol produced by hydrolysis of the alcoholate to be removed either by volatilization at ordinary temperatures or by dissolution in aqueous liquids with which the surface may be brought into contact.

PreferabIy theIefOre, the alcohols from which the aluminum alcoholates of the invention are prepared are the saturated aliphatic alcohols containing not more than 4 carbon atoms in the molecule such as methanol, eth anol, n-propanol, isopropanol and the normal, secondary, iso and tertiary butanols, and the dihydric and polyhydric aliphatic alcohols containing not more than 4 carbon atoms in the molecule such as ethylene glycol and glycerol.

The aluminum alcoholates of the invention may be prepared by the following general method:

Clean, dry aluminum (powder, turnings or strip) is heated under reflux with an excess of the appropriate alcohol and with exclusion of moisture, until solution of the metal is complete. Alternatively, the calculated amount of the alcohols together with an inert solvent such as benzene, toluene or xylene may be employed. If necessary, the aluminum may be activated by the addition of a few crystals of iodine or mercuric chloride to the reaction mixture. Inert impurities in the aluminum may be removed from the product by filtration and the alcoholates (with the exception of the methoxide) may be obtained in the pure state by distillation under reduced pressure.

The solvents commonly employed for alcoholates are 4: anhydrous aliphatic alcohols or ketones or aromatic hydrocarbons.

The following example more particularly describes the preparation of aluminum ethoxide:

Coarse aluminum powder (100 parts by weight) is covered with 650 parts by volume of dry xylene in a jacketed vessel fitted with a reflux condenser and a valved inlet. The xylene is heated to boiling and 440 parts by volume of absolute ethyl alcohol containing in solution 0.5 part by weight of mercuric chloride and iodine are added slowly to the boiling liquid. A vigorous reaction sets in necessitating cooling of the vessel. Further alcohol is added as required until about 350 parts by volume have been added when the vessel is again heated and the rest of the alcohol added slowly. The operation requires about 2 hours. When evolution of hydrogen has ceased, the reaction mixture is filtered hot to remove excess aluminum. The filtrate may be used as such in the process of the invention. Alternatively, the filtrate may be distilled to remove the solvent, and the aluminum ethoxide then dissolved for use in some other suitable medium such as a lower aliphatic alcohol, ketone or ether.

The following examples illustrate the process of the present invention:

Example I The process of the present invention when applied to aluminum coated metals, particularly to those where the coating has been applied by spraying, is found to increase the protection atforded to the metal base to a marked degree. This is illustrated by the following results of experiments in which specimens of mild steel coated with aluminum by spraying were compared with similar samples which had been treated further by painting, spraying or dipping with a 10% solution of aluminum isopropoxide in xylene according to the process of the invention. The samples were exposed to the products of combustion of a residual fuel oil at 800 C., for 100 hours. The fuel oil used had a viscosity of about 2000 seconds Redwood I at 100 F. and an ash content of 0.04% by weight. The ash contained, in per cent. by weight, 62.5 vanadium, calculated as vanadium pentoxide, 12.5 sodium, calculated as oxide and 7.5 calcium, calculated as calcium oxide.

Aluminum sprayed mild steel treated Aluminum sprayed mild steel with aluminum isopropoxide Covered with a layer of adherent scale with underlying iron aluminide layer.

Slight pitting.

Coating of alumina.

Badly scaled with underlying traces of iron aluminide.

Fairly extensive pitting.

Etched hollows contained traces of alumina which had absorbed some fuel-oil ash.

Protection afiorded by the coating was below the standard required.

Protection considerably better than in the sprayed only specimen and was of the requisite standard.

Example 1'] Iron castings which have been spray surfaced with aluminum, are subsequently sprayed with aluminum methylate so as to provide a continuous coating thereof. The castings are then exposed to air saturated with water vapor at C. until the methylate has been entirely converted to aluminum hydroxide. When castings so treated are exposed to air saturated with water, and compared with castings which have not been treated with aluminum methylate, it is found that the untreated casting shows serious rusting within a period of about 24 hours while the casting bearing aluminum hydroxide coating is sub stantially unaffected for periods of at least one week.

Example Ill Aluminum phenate is prepared by reaction of aluminum with phenol in benzene solution using substantially the same conditions as described hereinbefore. Iron blades which have been coated with an aluminum paint show evidence of rusting when exposed to air saturated with moisture at temperatures at about 90 C. in a period of approximately one day. When such blades are coated with aluminum phenate, subsequently heated to a term perature of about 400 C. for a period of two hours, the phenate is converted to a substantially continuous coating of aluminum oxide. Blades so treated show no evidence of rusting When exposed to Water saturated air at 90 C. for periods of at least one week.

We claim as our invention:

1. The process of protecting porous surfaces of aluminum clad corrodible metal articles, which comprises the steps of coating said surfaces with a solution of an aluminum alcoholate, evaporating the solvent from said coatings and hydrolyzing the alcoholate, whereby a substantially continuous coating of aluminum hydroxide is formed on the surfaces.

2. The process for protecting ferruginous metals bearing a porous aluminum surface which comprises coating said surface with an aluminum alcoholate dissolved in a relatively volatile solvent, evaporating said solvent and hydrolyzing the alcoholate, whereby a substantially continuous coating of aluminum hydroxide is formed, said coating sealing the porous aluminum surface.

3. The process for protecting ferruginous metals from corrosion and oxidation comprising spraying said metal with molten aluminum, whereby a relatively porous aluminum surface is formed, coating said surface with an aluminum alcoholate dissolved in a relatively volatile solvent, evaporating the solvent and hydrolyzing the alcoholate, whereby a substantially continuous aluminum hydroxide coating is formed, said coating sealing the pores in the porous aluminum surface.

4. The process for the protection of corrodible metallic articles bearing porous aluminum surfaces which comprises the steps of coating said surfaces with an aluminum alcoholate and converting said alcoholate to form an aluminum oxide, whereby a substantially continuous coating of alumina is formed and the porous aluminum surface is sealed.

5. The process for the protection of ferruginous metals which comprises spraying said metal with molten aluminum, whereby a porous aluminum surface is formed, coating said surface with an aluminum alcoholate and heating the coated and surfaced metal to a temperature at least sufiicient to convert the alcoholate to alumina, whereby a substantially continuous coating of alumina is formed on said aluminum surface.

6. The process for the protection of corrodible metals bearing sprayed aluminum surfaces comprising depositing on said surfaces a coating of an aluminum alcoholate dissolved in a polar organic solvent having less than about 9 carbon atoms per molecule, evaporating said solvent and converting the alcoholate to alumina, where by a substantially continuous coating of alumina is formed on the surfaces.

7. The process for the protection of corrodible metals bearing sprayed aluminum surfaces comprising depositing on said surfaces a coating of an aluminum alcoholate, each alcoholate radical of which contains less than 5 carbon atoms, and converting said alcoholate'to an aluminum oxide whereby a substantially continuous coating of alumina on said surface is formed.

8. The process according to claim 3 wherein the solvent is an aromatic hydrocarbon having less than 8 carbon atoms per molecule.

9. The process according to claim 8 wherein the solvent is xylene.

10. The process according to claim 7 wherein the alcoholate is aluminum isopropoxide.

11. The process according to claim 3 wherein the alcoholate is aluminum phenate.

References Cited in the file of this patent UNITED STATES PATENTS 346,345 Ball July 7, 1886 2,154,603 Bley Apr. l8, 1939 FOREIGN PATENTS 249,544 Great Britain Mar. 18, 1926 125,450 Australia Sept. 12, 1947 

1. THE PROCESS OF PROTECTING POROUS SURFACES OF ALUMINUM CLAD CORRODIBLE METAL ARTICLES, WHICH COMPRISES THE STEPS OF COATING SAID SURFACES WITH A SOLUTION OF AN ALUMINUM ALCOHOLATE, EVAPORATING THE SOLVENT FROM SAID COATINGS AND HYDROLYZING THE ALCOHOLATE, WHEREBY A SUBSTANTIALLY CONTINUOUS COATING OF ALUMINUM HYDROXIDE IS FORMED ON THE SURFACES. 